1. Field of the Invention
The present invention relates to a stacked radio-frequency module, and in particular, to improvements in the structure of a stacked radio-frequency module mounted in communications equipment such as radar or the like.
2. Description of the Related Art
FIGS. 13 and 14 are respectively a top view and a side cross sectional view of a conventional radio-frequency module. In FIG. 13, hatchings are drawn in certain sections in order to facilitate identification of each element. In FIG. 14, on the other hand, hatchings are not drawn in the cross sections for easier view. FIGS. 13 and 14 show a dielectric substrate 20, input/output pads 2 for radio-frequency signals (hereinafter referred to as xe2x80x9cRF padsxe2x80x9d), input/output pads 4 for power supply/control signals (hereinafter referred to as xe2x80x9cDC/CONT padsxe2x80x9d), via holes 7 within the dielectric substrate 20, strip wiring paths 8 for radio-frequency signals (hereinafter referred to as xe2x80x9cstrip wiring pathsxe2x80x9d), active microwave circuits 9 (hereinafter referred to as xe2x80x9cMMICxe2x80x9ds (Monolithic Microwave Integrated Circuit)), wiring paths 10 for control signals, and a metal sealing lid 12. The multi-layer substrate 20 has a structure scraped inside such that the MMICs 9 can be stored inside, and forms a cavity structure along with the metal sealing lid 12. FIGS. 13 and 14 further show ground potential surfaces 11 of the module, bonding wires 13 (hereinafter referred to as xe2x80x9cwiresxe2x80x9d), wire bonding surfaces 16 for radio-frequency signals and for power supply/control signals, and a mounting surface 17 for the metal sealing lid 12.
Next, the operation of such a conventional radio-frequency module will be explained. A radio-frequency signal input from the strip wiring path 8a is connected via a bonding wire 13a to an RF pad 2a and is transmitted to an MMIC 9a. The radio-frequency signal is then subjected to signal modulation by the MMIC 9a, such as, for example, amplification, attenuation, and phase shift of the signal, and transmitted to an MMIC 9b through an RF pad 2b, a wire 13b, and an RF pad 2c. The radio-frequency signal is then subjected to a signal modulation in the MMIC 9b similar to that in the MMIC 9a. The radio-frequency signal is further supplied to an MMIC 9c through an RF pad 2d, a wire 13c, a strip wiring path 8b, a wire 13d, and an RF pad 2e, and is modulated at the MMIC 9c. After this process, the radio-frequency signal is transmitted through an RF pad 2f, a wire 13e, and an RF pad 2g to an MMIC 9d, and is modulated at the MMIC 9d. Finally, the radio-frequency signal is output outside the module through an RF pad 2h, a wire 13f, and a strip wiring path 8c. 
Power supply/control signals input from a plurality of DC/CONT pads 4a, on the other hand, are transmitted through respective wiring paths 10a, 10b, 10c, and 10d for control signals which passes through the dielectric substrate 20, and wires 13g, 13h, 13i and 13j, to DC/CONT pads 4b, 4c, 4d, and 4e, to operate the MMICs 9a, 9b, 9c, and 9d. The ground potential surfaces 11 are grounded by a plurality of via holes 7, to set the ground potentials for the MMICs 9a 9d. 
Here, the MMICs 9a and 9b and MMICs 9c and 9d are respectively stored within two cavities formed by the dielectric substrate 20 and electromagnetically shielded by the metal sealing lid 12. To prevent erroneous operations due to signal interference, the MMICs 9a and 9b and the MMICs 9c and 9d are separated spatially and according to radio frequencies.
However, with such conventional radio-frequency module, there is a problem in that increase in the module size cannot be avoided because of the number and size of the MMICs 9 necessary for the function of the module. In recent years, there is a tendency for the module size to increase because of the higher demands for more functions in a module. On the other hand, there is a conflicting desire that the size of the radio-frequency module be reduced in order to respond to the increasing demand for reduction of module size to allow the use of signals having higher frequency, and increasing demands for reduction of the size of wireless devices.
A conventional stacked module which has been proposed in order to reduce the above described problems associated with the increased number of functions and size reduction will now be explained. FIG. 15 is a cross sectional view of this improved stacked module and FIG. 16 is a cross sectional enlarged view of a stacked bump section. In both FIGS. 15 and 16, the hatchings indicating the cross section are omitted for the sake of clarity. FIGS. 15 and 16 show stacked bumps 23, strip wiring paths 8, active semiconductor chips 9, dielectric substrates 20, an exposed section 21 of the strip wiring path, and a package 22. In this conventional example, a stacked module having a three-stage structure, in which three dielectric substrates 20a, 20b, and 20c are stacked, is shown. The dielectric substrates 20a, 20b, and 20c are collectively and air-tightly sealed by the package 22.
Next, the operations in this conventional example will be described. A strip wiring path 8a provided on the dielectric substrate 20a is connected, via stacked bumps 23, to a strip wiring path 8b provided on the dielectric substrate 20b at one stage above the dielectric substrate 20a. A strip wiring path exposure section 21 is formed on the strip wiring path 8b in which the dielectric substrate 20b is removed from the portion corresponding to the portion of the strip wiring path 8b to which the bumps 23 are to be connected. With such a structure, it is possible to connect the signal lines for the dielectric substrates 20a and 20b. Three dielectric multiple-layer substrates are stacked in the conventional stacked module, so that the stacked module has advantages that the mounting area for the module is extensively reduced, and, consequently, that the module size can be reduced.
However, because such a stacked module are developed for packages for storing active elements having relatively low operation frequency, such as a memory, the following problems are present when such a stacked module is employed as a radio-frequency module.
First, although the signal lines of the dielectric substrates are connected by stacked bumps, connection for ground signals, which is the counterpart to the signal lines, are not present between the stages, and, therefore, it cannot be assured that the radio-frequency signals will be reliably transmitted.
Second, because the clearance between the MMIC and the dielectric substrate at one stage above are determined only by the height of the stacked bumps, it has been difficult to secure sufficient clearance for mounting radio-frequency circuits. Because of this, the operation of the MMIC is influenced by the dielectric substrate at one stage above, and therefore, there is a problem in that desired characteristics may sometimes not obtained.
Also, because the shielding of the radio-frequency signals propagating through each stage is not sufficiently considered, the isolations between the stages are not sufficient. Because of this, there is a problem in that the signals to be transmitted may sometime interfere with each other, and smooth, reliable operation cannot be obtained.
Moreover, in order to connect the strip wiring path at the upper stage and the bumps, there is a need to expose the wiring path by precisely removing a portion in the dielectric substrate. Consequently, there is a problem in that a high-level manufacturing process is required and the cost is increased.
The present invention was conceived to solve the problems in the related art and one object of the present invention is to provide an improved stacked radio-frequency module having high functionality, a reduced size, and wider bandwidth.
In order to achieve at least the object mentioned above, according to the present invention, there is provided a stacked radio-frequency module formed by stacking dielectric multi-layer substrates onto which radio-frequency circuits are provided, wherein, at least two of the dielectric multi-layer substrates comprise a substrate having dielectric walls provided on its periphery; a closed space in which the radio-frequency circuits are stored, said closed space formed by being surrounded by the dielectric walls and the dielectric multi-layer substrate provided as an upper stage; an input/output terminal for radio-frequency signals provided on at least one of the upper surfaces of the dielectric walls and the bottom surface of the substrate at a position where the dielectric walls are provided, at a position opposing an input/output terminal for radio-frequency signals of the dielectric multi-layer substrate at the upper stage or lower stage; an input/output terminal for power supply/control signals provided on at least one of the upper surfaces of the dielectric walls and the bottom surface of the substrate at a position where the dielectric walls are provided, at a position opposing an input/output terminal for power supply/control signals of the dielectric multi-layer substrate at the upper stage or lower stage; a transmission path within the substrate for radio-frequency signals provided inside the dielectric multi-layer substrate, for connecting the input/output terminal for radio-frequency signals and the radio-frequency circuits; and a transmission path within the substrate for power supply/control signals provided inside the dielectric multi-layer substrate, for connecting the input/output terminal for power supply/control signals and the radio-frequency circuits, and gold bumps are provided for joining the input/output terminals for radio-frequency signals that are provided at opposing positions and for joining the input/output terminals for power supply/control signals that are provided at opposing positions.
According to another aspect of the present invention, it is preferable that the stacked radio-frequency module further comprises a dielectric multi-layer substrate for storing a control circuit for setting power supply/control signals for the radio frequency circuits.
According to another aspect of the present invention, it is preferable that a sealing lid for sealing the circuits is provided for the closed space.
According to another aspect of the present invention, it is preferable that a large-capacitance capacitor be provided on the bottom surface of the dielectric multi-layer substrate forming the closed space.
According to another aspect of the present invention, it is preferable that at least one of an external input/output terminal for radio-frequency signals and an external input/output terminal for power supply/control signals is provided on a side surface of a dielectric wall of the dielectric multi-layer substrate.
According to another aspect of the present invention, it is preferable that the input/output terminal for radio-frequency signals and the input/output terminal for power supply/control signals are provided on a surface created by partially removing at least one layer which forms the dielectric multi-layer substrate.
According to another aspect of the present invention, it is preferable that an electric wave absorber is provided in the closed space.
With the present invention, because gold bumps having small frequency characteristics are used for transmission paths of signals provided within the dielectric multi-layer substrates, a compact, multi-functional module which can operate at radio-frequency can be assembled. Also, because the joining surfaces by the gold bumps are provided at opposing end surfaces of two dielectric multi-layer substrates, partial substrate machining or the like for signal connection is no longer necessary, resulting in improvements in productivity of each of the packages having each of the dielectric multi-layer substrates. Moreover, a parasitic inductance component caused by the connection can be reduced with the use of the gold bumps. Further, because the structure is such that one vertical feed wiring path penetrates through different dielectric multi-layer substrates, signal transmission between the packages with wide bandwidth and low loss can be realized.
According to the present invention, because the distance between the sealing lid and the radio-frequency circuit can be secured by simply adjusting the height of the dielectric walls provided inside the dielectric multi-layer substrates, radio-frequency circuits can be designed with relative freedom and without large changes in characteristics.
According to the present invention, because the structure is such that the closed space in which radio-frequency circuits are installed is provided for each package so that each of the radio-frequency circuits are spatially completely separated, and, because no electromagnetic bonding or the like is used for signal transmission, it is possible to increase the electrical isolation between the radio-frequency circuits.
According to the present invention, because the control circuit is stored within a package, increase in the module size can be prevented.
According to the present invention, because the structure is such that the electromagnetic shielding is provided by air-tightly sealing the radio-frequency circuits by a sealing lid, it is possible to prevent leakage of radiation signals from the connection between the radio-frequency circuits and bonding wires. In this manner, erroneous operations caused by radiation signals can be prevented.
According to the present invention, because it is possible to provide large-capacitance capacitors within the closed space, it is possible to improve the stability of the radio-frequency circuits at low frequencies in cases where the radio-frequency circuits include an HPA or the like. It is also possible to increase the rise speed of the pulse waveform. Also, because the capacitors can be built into the module, the module can be maintained at a compact size.
According to the present invention, because an external input/output terminal for radio-frequency signals or for power supply/control signals is provided on a side surface of a dielectric wall of dielectric multi-layer substrates, it is possible to secure an area for radiation when a heat generating element such as an HPA is mounted in the lower-most layer, and, at the same time, the lengths of wiring paths propagating through the multi-layer substrates from the external input/output terminal to the radio-frequency circuits, such as the vertical feed line and the strip wiring paths, can be reduced. In this manner, loss involved with signal propagation can be minimized.
According to the present invention, because a portion of the dielectric multi-layer substrate is removed and input/output terminals for the signals are provided on the surface created by such a removal, it is possible to prevent degradation of characteristics due to excessive collapse of gold bumps upon joining of the dielectric multi-layer substrates by the bumps.
According to the present invention, because an electric wave absorber is provided within the closed space, attenuation of the signals leaking from the gold bump for connecting two dielectric multi-layer substrates forming the closed space and propagating through the closed space can be increased. Also, even when two signals transmitting through spatially separated gold bumps have different amplitudes, no unnecessary oscillation is generated, and, thus, the module can be operated without disadvantageous effect.